Breast tumor cells can disseminate prior to significant primary tumor growth and remain dormant in distant tissues for extended periods of time (Naumov, MacDonald et al. 2001; Naumov, MacDonald et al. 2002; Schmidt-Kittler, Ragg et al. 2003). Survival, invasion and reemergence of such disseminated cells are primary determinants of tumor recurrence and patient death (Chambers, Groom et al. 2002). Detachment of epithelial cells from the extracellular matrix of their organ of origin causes cell rounding that leads rapidly to apoptotic cell death, a principle which is thought to limit metastatic spread (Frisch and Francis 1994; Reed 2003). In mammary epithelial cells, we have shown that apoptotic resistance allows cells to survive rounding, but additional genetic mutations are required for active tumor growth (Martin and Leder 2001; Martin, Ridgeway et al. 2004; Pinkas, Martin et al. 2004). Resistance to apoptosis by overexpression of survival proteins, like Bcl-2, prevents cell death during dissemination, but cell cycle arrest can still occur through activation of p53 (Nikiforov, Hagen et al. 1996; Nikiforov, Kwek et al. 1997). In solid tumors like breast cancer, detached cells generally remain arrested and must adhere to extracellular matrix in distant tissues to reinitiate growth (Naumov, MacDonald et al. 2001; Naumov, MacDonald et al. 2002). So while apoptotic resistance can promote extended bloodborne survival, additional mechanisms are required for tumor cells to escape blood vessels and successfully colonize distant tissues. (Naumov, MacDonald et al. 2001).
In vivo microscopy recently demonstrated that bloodborne tumor cells depend on tubulin polymerization to attach to the walls of capillary blood vessels (Korb, Schluter et al. 2004). However, any specific role for microtubules in this process remains unclear. Surprisingly, this recent study also showed that inhibiting actin polymerization greatly increased binding of tumor cells to blood vessel walls, even though actin depolymerization inhibits the actin-based invadopodia and podosomes that are well-known to affect the invasion of adherent tumor cells (Korb, Schluter et al. 2004). Bloodborne tumor cells therefore attach to capillary vessel walls via a cytoskeletal mechanism that is distinct from that of adherent cells, and is currently not well-characterized.
Recent genomic studies indicate that breast tumor cells may disseminate prior to significant primary tumor growth and remain dormant in distant tissues for extended periods of time (Schmidt-Kittler et al., 2003). Detachment of epithelial cells from the extracellular matrix of their organ of origin leads rapidly to apoptotic cell death, a principle which is thought to limit metastatic spread (Valentijn et al., 2004; Reddig and Juliano, 2005; Frisch and Francis, 1994). In mammary epithelial cells, apoptotic resistance will promote the survival of detached cells, but additional genetic mutations are required for active tumor growth (Pinkas et al., 2004; Martin et al., 2004). Resistance to apoptosis by overexpression of survival proteins, like Bcl-2, prevents cell death during dissemination, but cell cycle arrest can still occur through activation of p53 (Nikiforov et al., 1996; Nikiforov et al., 1997). Since these cells would survive but fail to actively grow, apoptotic resistance is one possible determinant of tumor dormancy (Townson et al., 2003).
However, evidence is accumulating that resistance to apoptotic cell death does not necessarily prevent early stress responses from occurring in cells. The apoptotically-resistant MDA-MB-231 cell line responds to hypoxic environments with increased invasiveness and the upregulation of cell surface α6β4 integrin (Yoon et al., 2005). Lymphocytic cell lines that overexpress Bcl-2 still generate reactive oxygen species in response to TNF-α, even though the later events in cell death are prevented (Liu et al., 2005). It is therefore important to consider whether apoptotically-resistant cells will truly remain dormant during the challenges of metastasis, or if they persistently respond to the challenging environment even though they are under a reduced threat of death.
Detachment of many adherent cell types from the extracellular matrix results in apoptotic cell death that arises from disrupted cell shape (see Valentijn et al, 2004; Reddig and Juliano, 2005 for review). In attached cells, tension generated by attachment of actin microfilaments to focal adhesions is counteracted by expansion of cytoplasmic microtubules to stabilize cell shape in a process termed tensegrity (Ingber, 2002; Wang et al., 2001). Disruption of this balance either by affecting cell attachment sites or directly inhibiting cytoskeletal structure induces rapid cell death (Chen et al., 1997; Martin and Leder, 2001). In order to survive detachment, epithelial cells must either avoid apoptosis or quickly reattach to adopt an appropriate cell shape. While tumor cells often develop resistance to apoptotic cell death, this phenomenon is rare in untransformed cells (Reed, 1999).
The present invention provides a long-felt solution to treating cancer, for example by preventing or reducing metastasis by targeting particular microtubule-associated structures heretofore unknown in the art.